The present disclosure relates to ring gears and methods for manufacturing a ring gear.
Conventionally, planetary gear units including a ring gear, which are used for automatic transmissions for vehicles etc., are known in the art. For example, Japanese Patent Application Publication No. H04-285346 (JP H04-285346 A) discloses such a planetary gear unit.
Japanese Patent Application Publication No. H04-285346 (JP H04-285346 A) discloses a planetary gear unit for use in automatic transmissions for vehicles, which includes a ring gear, a pinion gear that meshes with internal teeth of the ring gear, and a sun gear that is disposed inward of the pinon gear and meshes with the pinion gear. Since the internal teeth of the ring gear are required to have predetermined hardness, the internal teeth of the ring gear in the planetary gear unit of Japanese Patent Application Publication No. H04-285346 (JP H04-285346 A) have been subjected to nitrocarburizing in order to improve hardness of the internal teeth. Induction hardening may be used to increase hardness of the internal teeth. However, since the processing temperature of nitrocarburizing is lower than that of induction hardening, distortion due to nitrocarburizing is restrained and the internal teeth can be formed with high accuracy. Gear noise that is generated by meshing between the pinion gear and the internal teeth of the ring gear is therefore reduced as compared to induction hardening.
In a planetary gear unit disclosed in Japanese Patent No. 4,867,430, a clutch that is a friction engagement element is disposed radially outward of a ring gear. The clutch includes outer thin plates (clutch plates) 43 that are first friction materials, inner thin plates (clutch discs) 44 that are second friction materials, etc., and splines formed in the outer peripheral surface of the ring gear R1 are engaged with splines formed in the inner peripheral surface of the clutch discs 44. The ring gear R1 is thus configured to serve also as a hub of a clutch Ca. Similarly, a ring gear R2 also serves as a hub of a clutch Cb.